Stripping methods for the removal of coatings applied on metal surfaces for instance by electroplating or PVD or CVD processes have been described in prior art. For example, TiN coatings are easily dissolved using an aqueous hydrogen peroxide solution. However, in applications where more complex layers are to be removed, such as ternary or quaternary coatings whose mechanical and/or chemical properties make them particularly suitable for many applications in the tool and die industry and in mechanical engineering, more complex methods are typically required. For example, one method requires the use of a variety of chemicals some of which are expensive and/or environmentally or toxicologically unsafe. Another example is by use of electrolysis or similar process that requires electrical contact with the work piece. In many cases, these more complex methods require unacceptable processing times in the manufacturing industry.
JP 3320965 discloses a method for stripping TiAlN, ZrAln, HfAlN as well as Si3N4 hard-material coatings. It employs alkaline solutions containing various concentrations of permanganate and dichromate ions. However, a satisfactory removal of the layers specified was not possible until relatively high dichromate concentrations along with a high pH value and temperatures above 40° C. were used or an electrolytic process was added. The fastest stripping times achieved were in the range from 1–5 hours. Yet dichromates are known to have a high toxic potential because of the hexavalent chromium, and their use as well as disposal requires special precautionary measures. Moreover, that method causes small pores to form in the substrate, intended to enhance the bonding of the coating. However, this is undesirable for polished substrates.
JP 02-285081 discloses a method for stripping chrome or chromium oxide coatings. The process involves an aqueous solution with the addition of a corrosive agent and an aromatic or fluoric tenside.
The German patent application DE 4339502 describes the nondestructive stripping of carbide substrates coated with among others TiAlN layers. The improvement over prior methods consists of, apart from the usual complexing agents and stabilizers, the use of anticorrosive inhibitors along with other process materials and adjusting the pH value of the solution which, in combination with the other reagents, prevents the separation of Co from the work piece. The drawbacks of that approach include the comparatively long stripping time for TiAlN and others, the relatively extensive and correspondingly expensive use of chemicals, the fairly complicated formulations and reaction conditions (that must be precisely observed), as well as the use of fluoric reagents.
WO9964646 describes a stripping method whereby a work piece is first coated with a thin TiN layer subsequently followed by the application of the hard-to-remove functional TiAlN layer. Stripping is performed using a hydrogen peroxide solution which, penetrating the pores in the cover layer, dissolves the intermediate TiN layer. The drawback of TiN coatings, however, is that they offer relatively low thermal stability compared for example to TiAlN or AlCrN. TiN coatings exposed to air develop a damaging oxidation process at temperatures as low as 600° C., which leads to the complete failure of the coating after a prolonged exposure. To remove the highly heat-resistant coatings after the detection of flaws in the manufacturing process or prior to the recoating of an expensive tool without damaging the delicate tool surfaces, a considerable number of stripping processes were developed, including complex and even electrolytic procedures like those mentioned in JP 3320965 or WO 1999-54528.
Thus, it is the objective of this invention to introduce a method for the stripping of hard coatings without the drawbacks of prior-art techniques. In particular, this method is intended to permit easy and rapid stripping using safe chemicals. Another objective of the present invention includes a stripping method for coatings that can also be used at extremely high operating and processing temperatures. For example, for TiAlN any oxidative damage would not occur at temperatures below approximately 800° C. However, the method lends itself particularly well to applications on AlCrN, Al2O3, (AlCr)2O3 or (AlCr)xOyNz layers where a failure of the coating or of the layer/substrate bond does not manifest itself until temperatures over 1000° C. are reached.